1. The Field of the Invention
The present invention is directed to a power supply, and more particularly to an inverter AC power supply providing a high frequency AC voltage to a load with an improved power factor.
2. Description of the Prior Art
In an inverter AC power supply for use to drive such as discharge lamps and electric motors, it is required to smooth an input voltage applied to the inverter by the use of a capacitor. It is known that the capacitor acts as a reactive load with a high capative reactance and therefore reduces a power factor. To eliminate the adverse effect of the capacitor, an inductor or choke coil may be coupled with the capacitor to reduce the capative reactance of the capacitor. However, due to the fact that a source voltage for the inverter is normally a commercial line voltage of a relatively low frequency, the inductor or choke coil is limited to have a high inductance for such low frequency and therefore bulky, which unduly increases the size of the power supply and therefore should be avoided for compact design of the power source. To this end, it has been proposed to chop the voltage applied to the inverter input at a high frequency to thereby permit the use of a choke coil of less inductance and size. However, such chopping operation requires an additional switching circuit to the inverter circuit and therefore adds complexity to the overall power supply circuit with considerable duplication of components. For simplifying the circuit and reducing the number of components, U.S. Pat. No. 4,564,897 proposes to use common component both to the inverter and chopper operations. Unfortunately, the patent is found unsatisfactory as the common components are limited to only one of plural switching elements and there are still a greater number of other components required separately for the inverter and chopper operations. Therefore, it is most desirous to share not only one switching element but also the other components as many as possible for further reducing the number of duplicated components and therefore the size and cost of the power supply.